A BMI Referendum

My print column examines the continued use of Body Mass Index in epidemiological studies and in clinical practice to screen for being overweight or obese. Some researchers say that while BMI improved on its predecessors, it fails to distinguish between different kinds of body mass and therefore can mislead about individuals’ health levels — a longstanding criticism of the measure that hasn’t prevented it from becoming the primary tool for grouping people into normal-weight, overweight and obese categories.

“When it’s applied to single individuals to make a statement [about their health], it’s nonsense,” said Stanford University Mathematician Keith Devlin. He has been rallying against BMI, and its classification of his competitive-cycling-shaped figure as overweight, for several years. He trained for a 130-mile, mountainous one-day race in 2009, and in the process added enough muscle mass to cross the overweight threshold, and was stunned to be counseled on weight loss by his doctor.

“To present it as number with an aura of science and mathematics, that, I think is dishonest,” Devlin said. “I don’t like my discipline being misused.”

Steven Heymsfield, executive director of the Pennington Biomedical Research Center, a National Institutes of Health-funded institution in Baton Rouge, La., said of BMI, “It is reliable as a measure of over-weight but not over-fat. There are both false positives and false negatives, people who are overweight but not over-fat and vice versa.” False positives are “not uncommon in certain populations, for example the military.”

Heymsfield estimates such false readings have a prevalence of 5% or 10% of the population, but it could be higher. It is possible, for instance, that enough healthy, muscular people are in the overweight category, and enough sick people who have lost weight from illness are in the normal-weight range, “that you could completely explain away these results,” said Heymsfield of results from a recent Journal of the American Medical Association-published study suggesting that people who are moderately overweight have less risk of dying in a given period than people of normal weight. “We don’t really know the true proportion of people falsely classified.”

Barry I. Graubard, a senior investigator at the NIH’s National Cancer Institute and co-author of the JAMA paper, agreed that “BMI is not the best measure of body fat. BMI is measure of body weight adjusted for height and is highly correlated with body fat.” Graubard added, “BMI is mainly used because it is convenient and inexpensive, requiring no elaborate instrumentation.”

Francisco Lopez-Jimenez, a cardiologist and professor of medicine at the Mayo Clinic in Rochester, Minn., said that “the introduction of BMI was in general a good thing.” Before its introduction, doctors and researchers had to consult clunky and flawed height-weight tables. The problem is how it BMI has been interpreted. “Most people have been confusing using something for epidemiological studies, and using something for medicine.”

BMI works best “as a screening tool, rather than a diagnostic one,” said Susan Yanovski, co-director of the Office of Obesity Research at the National Institute of Diabetes and Digestive and Kidney Diseases, part of NIH. “The NIH is encouraging research on more accurate, inexpensive, and portable measures of body composition that can be used in clinical and population studies.”

The National Heart, Lung, and Blood Institute, which adopted BMI as the standard guideline for obesity in 1998, is sponsoring reports that “will examine the evidence related to BMI,” said a spokeswoman for NHLBI, a part of NIH. “Because the NIH is aware of the questions surrounding BMI, we have funded and are continuing to encourage research on more accurate, inexpensive, and portable measures of body composition that can be used in both clinical and population studies.”

BMI’s advantage is that it “is a low-cost measure that can be obtained quickly,” said Judith Wylie-Rosett, division head for health promotion and nutrition research at Albert Einstein College of Medicine in Bronx, N.Y. And changing clinical practice to a different measure is difficult. For instance, despite recommendations that more doctors measure patients’ waist circumference for evidence of obesity, “for some reason, it’s just not done,” Wylie-Rosett said. “We’ve trained medical students, with tape measures.” She added, “We need to get more sophisticated in how we do the assessment but we need to keep it low-cost

David Dunson, a biostatistician at Duke University, questions his own BMI reading since it classifies his muscular frame as overweight. However, he acknowledges the usefulness of having “a one-number summary that is simple to calculate and it is not clear that there is a better summary than BMI — at least based on using only height and weight.”

Some researchers advocate supplementing or supplanting BMI with waist circumference, a more direct measure of abdominal fat, believed to be the most dangerous kind of body fat. “You can get a better fix by also including waist-circumference measures,” Heymsfield said. “They can put you into groups of high and low risk in the overweight range. I think it’s a good second step.”

However, Walter Willett, chair of the department of nutrition at the Harvard School of Public Health, cautions that while waist circumference is “conceptually better and more specific” than BMI, it is “measured less precisely” — for instance, not every doctor will squeeze the tape measure to the same tautness.

And BMI does have its defenders. “For the vast majority of people, BMI works very well as an indicator of body fatness,” said Frank Hu, professor of nutrition and epidemiology at the Harvard School of Public Health. “Both BMI and waist circumference are useful markers of obesity risk and these measures are complementary to each other.”

About The Numbers

The Wall Street Journal examines numbers in the news, business and politics. Some numbers are flat-out wrong or biased, while others are valid and help us make informed decisions. We tell the stories behind the stats in occasional updates on this blog.